This invention relates to a method and apparatus for placing and attaching solder balls to substrates, particularly ball grid array (BGA) packages) without using flux.
Present methods used for attaching solder balls to BGA packages employ flux, a material which prevents the formation of oxides during reflow of the solder balls. In a typical "flux" method, the surfaces of the conductive pads of the substrates are coated, e.g., screened, with flux and the balls are placed into position on the package. As the flux dries, the balls become stuck to the pad surfaces. Occasionally, two balls may stick to a single pad surface or, conversely, no balls may stick to a particular pad surface. The assemblies are placed on a conveyor and the balls are reflow-soldered to the BGA pads in a furnace. As the assemblies heat-up during the reflow process, the flux is liquefied and the flux bonds between the balls and pads are destroyed. As a result, the balls are free to move during the rigors of transport through the furnace. Consequently, defects such as vacancies, bridging of adjacent balls and loss of positional accuracy are created. Also, flux residues are trapped between the pads and the underside surfaces of the balls and is difficult to completely remove. These uncleaned residues may, depending on the type of flux used, cause high resistance shorts and/or corrosion subsequent to final assembly soldering operations. Consequently, the flux residues must be removed which requires a separate cleaning step and apparatus. Moreover, many current cleansers used to remove flux are not environmentally safe. U.S. Pat. Nos. 5,499,487 and 5,551,216, both of which are incorporated herein by reference, disclose conventional methods and devices for the placement of solder balls on BGA packages.
The demand for BGA devices has steadily increased, and more efficient high volume manufacturing assembly methods need to be developed to keep pace. Production yields also need to be improved so that the cost of manufacturing can be reduced. Accordingly, there is a need for a new and more cost effective method of placing and attaching solder balls on BGA packages.